CancerQuest | Cells of the Acquired Immune Response

The main cells of the specific immune response are lymphocytes - B cells and T cells. All lymphocyte precursors originate in the bone marrow. The pre-B cells stay in the bone marrow to undergo further development, while the T cell precursors migrate to an immune organ located in the neck (the thymus) to further develop. In fact, T cells get their name from the thymus. For trivia buffs: B cells are named after an organ found in chickens (the bursa of Fabricius) where they were first studied. Humans do not have an equivalent organ.

Early in T cell and B cell development, developing cells that strongly react with normal cell proteins are removed from the system. In this way, the immune system ensures that the B cells and T cells do not kill normal body cells. If self-reactive T cells and B cells are not removed from the lymphocyte population, autoimmune diseases like lupus or rheumatoid arthritis may develop.

There are two classes of mature T cells:

Helper T cells- These cells help other immune cells, including CTLs, macrophages and B cells, carry out their functions more efficiently.
Cytotoxic T Lymphocytes (CTL)-(cyto=cell and toxic because they can kill) These are cells that are able to kill other cells, they are cellular assasins. They directly kill any cell that they recognize as abnormal, such as cells infected with viruses or cancer cells.

The immature T cells residing in the lymph nodes and spleen do not mature into full effector cells until an APC comes to them and shows them, or presents to them, a particular protein antigen. Once the T cell is notified by the APC that there are cells in the body expressing these abnormal proteins, the T cells mature and leave the lymph nodes and the spleen to circulate in the body and find the abnormal cells. When the T cells find the abnormal cells they are able to kill them. In the case of virus infection, killing the infected cell is a harsh but effective way to limit the production of the viruses within. Cancer cells may also be recognized and eliminated by cytotoxic cells of the immune system.

B cells are another critical component of the acquired immune response. Like T cells, B cells are formed in the bone marrow. The cells move out into the body to mature. B cells are responsible for producing antibodies, proteins that recognize foreign objects that enter the body (viruses, bacteria, other proteins, etc.). Different B cells can recognize different targets. There are millions of different kinds of B cells in our bodies and our immune system can respond to a very large number of different 'foreign' targets.

The immune system functions as an effective surveillance system to eliminate abnormal cells and invading organisms from our bodies.